The present application is related to machine vision vehicle service systems, such as a vehicle wheel alignment system, and in particular, to methods for determining the physical relationship between camera coordinate systems of multiple cameras having overlapping fields of view, to methods for identifying changes in optical components of the machine vision vehicle service system after calibration, and to methods for evaluating in total, the optical performance of a machine vision vehicle service system, the optical performance of groups of imaging components within the machine vision vehicle service system, and the optical performance of individual imaging components.
Machine vision vehicle wheel alignment systems utilize imaging sensors or cameras to measure the position and attitude of targets mounted within observable fields of view. These targets may be mounted to individual wheels of a vehicle, to components or structures associated with a vehicle, or to fixtures or devices utilized for calibration purposes. As used herein, the term “camera” may be used interchangeably with “imaging sensor” and “imager”. The vehicle wheel alignment systems are configured to process the target images to determine, for each observed target, a target position and attitude, expressed in a coordinate system associated with the observing camera. In order for the vehicle wheel alignment system to complete a computation of vehicle alignment angles involving multiple wheels of the vehicle, knowledge of the physical relationships between each of the observing camera coordinate systems is required.
Typically, the cameras of a machine vision vehicle wheel alignment system are organized as a left camera assembly and a right camera assembly (each disposed to observe opposite sides (left and right) of a vehicle longitudinal centerline). In some configurations, one camera assembly is mounted to each end of a camera crossbeam that is supported by a mounting structure. Alternatively, each camera assembly is disposed in an independent pedestal, with a field of view directed at the associated side of a vehicle. In a four-camera configuration, each camera assembly includes a first camera, referred to as a “front camera” that is configured with a field of view to acquire images of a target mounted on a front wheel of the vehicle, and a second camera, referred to as a “rear camera” that is configured with a field of view to acquire images of a target mounted on a rear wheel of the vehicle on the same side as the front wheel observed by the first camera. Alternative configurations may only utilize two cameras, one in each camera assembly, and each of which is configured to acquire images of targets mounted on both the front and rear wheels of the vehicle.
Within a four-camera configuration, the relationship between the two cameras mounted in a camera assembly is called the rear-to-front cross transform. Traditionally, this relationship is established and measured in a factory at the time of assembly, and is unchanged during the operational life of the camera assembly. The process for establishing the relationship is commonly referred to as rear-to-front cross-calibration, and the resulting rear-to-front cross transform is stored in non-volatile memory within one or both cameras of the two-camera assembly. Because the rear-to-front relationships and cross transforms were set at the factory, field repairs of a failed camera in a machine vision alignment system with a four-camera configuration necessarily required replacement of the entire two-camera assembly instead of merely replacing an individual failed camera.
The computation of vehicle wheel alignment angles requires knowledge of the physical relationships between all of the cameras utilized by a machine vision vehicle wheel alignment system, as well as the relationship between the cameras and the surface of the vehicle lift or “runway” on which the vehicle is resting. Although rear-to-front relationships were determined in the factory, the remaining relationships were determined at the vehicle service site in a process referred to as “field calibration”, such as shown in U.S. Pat. No. 7,453,559 to Dorrance et al. herein incorporated by reference.
The process of calibrating a machine vision vehicle wheel aligner usually involves a special procedure executed by a service technician and directed by the computer software of the aligner's “console” computer. This procedure requires the computer to record a series of observations of targets mounted to a portable fixture, such as a calibration fixture, placed in a series of positions on the alignment runway or vehicle support surface.
The fixture is a stand that supports a long bar horizontally across the runway or vehicle support surface. Near each end of the fixture is a saddle-like bearing that allows the horizontal bar to rotate around a horizontal axis. At each ends of the rotating bar is a socket. The presence of these sockets allows the mounting of a machine vision target on each end of the bar, such that a target is disposed within the field of view of an associated camera assembly on each respective side of the runway.
Each camera assembly is pointed so that the cameras collectively view a volume of space that runs along each side of the runway. The collective field-of-view of the left camera assembly does not overlap with the collective field-of-view of the right camera assembly, however, some degree of overlap is present for the fields of view of the front and rear cameras on a common side of the runway. During calibration procedures, the calibration fixture provides a physical connection between a target that is visible within the field of view a left side camera assembly and a target that is visible within the non-overlapping field of view of a right side camera assembly. When a series of observations are gathered involving a variety of calibration bar orientations, positions, or rotations, there is sufficient information to mathematically estimate a transform between camera assemblies for viewing the left and right sides of a vehicle, and cameras for viewing front and rear wheels of a vehicle, each of which may have non-overlapping fields of view.
The relationship between cameras and the runway surface can be measured by taking observations of the calibration fixture standing in a front axle area of the runway plus observations of the same fixture standing in a rear axle area of the runway surface. The observations allow the computation of the relative heights of the points where the calibration fixture was supported on the runway surface, which can lead to knowledge of the orientation of a “runway plane” relative to the coordinate system of a master camera.
After observations are gathered, a mathematical algorithm is executed by the alignment software that produces an estimate of physical relationships between certain pairs of cameras and an estimate of the physical relationship between a camera and the runway surface. These physical relationships between cameras or between a camera and the runway are often called “transforms”. The set of observations must provide the computer with sufficient evidence for it to be able to unambiguously determine those transforms needed to complete the geometric information necessary to compute vehicle wheel alignment angles.
The traditional field calibration procedure for a four camera vehicle wheel aligner determines the transform between the right front camera and the left front camera, plus the transform between the left front camera and the runway. The observation set needed to compute these transforms involves a series of calibration fixture positions in the front axle area of the runway surface, plus a series of calibration fixture positions in the rear area of the runway surface. There are front observations seen only by front cameras plus rear observations seen only by rear cameras. Computation of the full set of inter-camera relationships relies upon the front-to-rear transforms for the right and left camera assemblies that were established at the factory during manufacture of the camera assemblies.
Accordingly, it would be advantageous to provide a process of field calibration for a four-camera vehicle wheel aligner which enables the rear-to-front relationships between the two cameras in an individual camera assembly to be determined at the vehicle service site, enabling in the field replacement of a single defective camera, and eliminating the need to replace an entire camera assembly.
The transforms which are determined from the calibration procedures remain valid so long as the physical relationships between the various components (i.e. imaging sensors) do not change, and so long as the optical properties of the elements comprising the components (i.e. targets, lenses, imaging sensor arrays, etc.) are not altered in some way. Accordingly, it would be advantageous to provide a method by which changes in optical components of the machine vision vehicle service system which occur after calibration has been completed may be identified. It would further be advantageous to provide a method by which the overall optical performance of a machine vision vehicle service system could be evaluated, as well as the optical performance of groups of imaging components within the machine vision vehicle service system or even of individual imaging components themselves.